docs: def Huq_Loth(x, rhol, rhog):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_, also given in [2]_, [3]_, and [4]_.
  
      .. math::
          \alpha = 1 - \frac{2(1-x)^2}{1 - 2x + \left[1 + 4x(1-x)\left(\frac
          {\rho_l}{\rho_g}-1\right)\right]^{0.5}}
  
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      [1]_ has been reviewed, and matches the expressions given in the reviews
      [2]_, [3]_, and [4]_; the form of the expression is rearranged somewhat 
      differently.
  
      Examples
      --------
      >>> Huq_Loth(.4, 800, 2.5)
      0.9593868838476147
  
      References
      ----------
      .. [1] Huq, Reazul, and John L. Loth. "Analytical Two-Phase Flow Void 
         Prediction Method." Journal of Thermophysics and Heat Transfer 6, no. 1 
         (January 1, 1992): 139-44. doi:10.2514/3.329.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      .. [4] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
         Fraction Correlations for Different Flow Patterns in Horizontal and 
         Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
         no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.
      '''
      B = 2*x*(1-x)
      D = (1 + 2*B*(rhol/rhog -1))**0.5
      return 1 - 2*(1-x)**2/(1 - 2*x + D)